1. Field of the Invention
The present invention relates to a point source for testing a gamma camera for field uniformity, spatial resolution and linearity.
2. Description of the Prior Art
A gamma camera is a diagnostic scintigraphic apparatus which is used in medical applications to monitor the progress or distribution of a gamma ray emitting radionuclide that is introduced into a patient. The nuclide of choice is technetium-99M. The gamma camera is positioned in adjacency to that portion of the body of the patient under examination. The camera includes a gamma ray sensitive solid state crystal which provides a response functionally related to the distribution of the nuclide in that portion of the patient's body upon which the camera is trained.
It is essential that the gamma camera undergo careful quality control calibration so that any nonlinearity or nonuniformity in the response of the camera is accommodated in the results of the diagnostic examination using the camera. Preferably the calibrating apparatus utilizes as a gamma ray source either the same nuclide as is used in a patient or a source able to emit gamma rays having similar energy level. The gamma ray emitted from technetium-99M is 140 KeV.
Typical modalities for calibrating gamma cameras include fillable phantom sources, flood sources and point sources. Representative of the structure of a flood source for calibrating gamma cameras is that shown in U.S. Pat. No. 4,033,884 (Lorch et al), U.S. Pat. No. 3,064,130 (Di Ianni et al) and U.S. Pat. No. 4,517,460 (Meulenbrugge et al) both relate to the calibration of gamma instruments.
Technetium-99M is commonly used for a point source modality but due to its six-hour half-life it requires daily preparation. Another material that may be used as a point source is tellurium-123M. However, tellurium-123M has a gamma ray at 159 KeV, which is spaced from that of technetium-99M. Cobalt-57, although used primarily for flood sources, may also be used in a point source, however, its primary gamma ray at 122 KeV is also spaced from that of technetium-99M.
Accordingly, in view of the foregoing, it is believed advantageous to provide a point source for calibrating gamma cameras which utilizes a radioactive material emitting a monoenergetic gamma ray closer to that of technetium-99M than that emitted by cobalt-57 or tellurium-123M. Further, it is believed desirable to provide a point source using a radioactive material that has a sufficiently long half-life to avoid repeated changes of calibrating source. The radioactive material should preferably be contained in a solid sealed member to maximize safety and convenience in use. It is also desirable to provide a packaging arrangement for the point source which will permit incremental exposure of the point source over a prolonged period (on the order of three months) whereby the same predetermined relatively constant gamma flux may be emitted from the source.